1. Field of the Invention
The present invention relates to an image reading apparatus and a control method therefor.
2. Description of the Related Art
Image reading apparatuses equipped with an automatic document feeder (ADF) are predominantly configured to read originals on a one side-by-one side basis. This type of image reading apparatus reads an image on the front side of an original while conveying the original, reverses the conveying direction of the original to invert the original by an inverting mechanism provided on the conveying path, and then reads the reverse side of the original while conveying the original again. Each original is thus conveyed to a reader unit twice, first with its front side facing the reader unit, and then with its reverse side facing the reader unit, whereby images on the respective front and reverse sides of the original are read.
The image reading apparatus of the above-mentioned type is required to perform an operation for reversing the original conveying direction after scanning the front side of an original, and an operation for inverting the original having its reverse side scanned so as to discharge the original from the apparatus in the same page order as before conveyance. Therefore, it takes time to read a double-sided original.
In order to reduce the time required for a double-sided original, there have been proposed image reading apparatuses in which two reader units are provided on a conveying path such that they can read the respective sides of an original without inversion of the original (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2002-135530).
Each of the reader units of an image reading apparatus of the above-mentioned type is comprised of a light source and a reading sensor. In the image reading apparatus, the upstream light source with respect to the flow of fed originals is first turned on and then the downstream light source is turned on, so as to shorten a time period over which the light sources are kept on, to thereby enhance the energy-saving effect of the apparatus.
In general, the reading characteristics of a reading sensor change due to environmental changes including the lapse of time after the start of driving and a rise in temperature. For this reason, it is required to carry out shading correction for correcting unevenness of the reading sensitivity of the reading sensor in the transverse direction thereof. Time period required for acquisition of correction data for shading correction differs depending on the type of a scanner and the warm-up time of a light source. The typical types of scanners include a CCD scanner type and a CIS scanner type. The CCD scanner type scans an image on an original by moving a lamp and a mirror along the original, and then reads the original image zoomed out by a lens by a charge coupled device (hereinafter referred to as “the CCD sensor”) thereof. On the other hand, the CIS scanner type scans an image on an original by moving a contact image sensor (hereinafter referred to as “the CIS”) thereof relatively to the original to thereby read the scanned image of the original.
The CCD sensor of the CCD scanner can be reduced in size because it is configured to read a zoomed-out original image, and therefore it is possible to form the CCD sensor by a single sensor chip. On the other hand, a reading sensor of the CIS in the CIS scanner is configured to perform a reading operation in a state brought into contact with an original, and hence the reading sensor needs the same length as the width of the original. For this reason, the CIS scanner is required to be formed by a plurality of (e.g. sixteen) sensor chips arranged in the transverse direction of an original. This necessitates correction of density differences at junctures between adjacent ones of the sensor chips, and therefore it takes a longer time to acquire correction data for the CIS scanner than for the CCD scanner.
Let it be assumed that the CCD scanner is disposed on the upstream side of CIS scanner. As shown in FIG. 8, in response to a job start instruction, the upstream light source is turned on so as to start acquisition of correction data for the upstream reading sensor, and then the downstream light source is turned on so as to start acquisition of correction data for the downstream reading sensor. Only after acquisition of the correction data on both the upstream and downstream reading sensor, an original reading operation can be started. In this case, it takes a longer time to acquire correction data for the downstream CIS scanner which is turned on later than for the upstream CCD scanner which is turned on earlier. Therefore, it takes a long time to complete acquiring correction data for both of the upstream and downstream scanners after the start of the job is instructed.